Mast cells are ubiquitously distributed throughout mammalian organs in association with connective tissue. Special consideration is being given to the role of mast cells in the lung where these cells are present beneath the epithelium in trachea and bronchi as well as in mucous lining. Mast cells contain bioactive amines such as histamine and serotonin, sulfonated glycosaminoglycans, e.g., heparin, and peptide chemotactic factors, as well as polypeptide hormones such as vasoactive intestinal polypeptide, corticotropin (ACTH) and beta-lipotropin (beta-LPH). This investigation then focuses on: (1) the characterization of the precursor of mast cell ACTH and its comparison to that from pituitary glands; (2) the in vitro synthesis of peptide hormones in mast cells; (3) the processing and release of ACTH and beta-LPH by bioactive agents and peptide hormones; and (4) the use of a mastocytoma cell line to study the effect of reversible granulopoiesis on the synthesis, storage and release of an enkephalin precursor. Studies using a mouse mastocytoma have shown that this tumor contains numerous polypeptide hormones. Therefore, it appears to have the capacity for ectopic synthesis of peptide hormones analogous to that observed in various pulmonary tumors.